It is a general and conventional practice to apply a coating on the inside and outside of a metal can to prevent corrosion, wherein the coating is generally made from a thermosetting resin.
For application of a thermosetting resin coating, a solvent type coating is used in most cases. The formation of a coating film requires heating at a high temperature for a long time, for example, 150-250° C. for several minutes. In addition, a large amount of the organic solvent scatters during baking, giving rise to a demand for an improvement including simplification of the process, prevention of pollution and the like.
The film formed under the aforementioned conditions inevitably contains a small amount of the organic solvent. When, for example, food is filled in a metal can having the above-mentioned film inside the can, the organic solvent may be transferred to the food to spoil the taste and flavor of the food. It also happens that the additive in the coating and a low molecular weight substance in the coating, which has been produced by incomplete crosslinking reaction, may be transferred to the food, thereby causing an adverse influence on the food, like the aforementioned residual organic solvent.
A different method for preventing corrosion is the use of a thermoplastic resin film. For example, a polyolefin film such as a polypropylene film and the like or a polyester film is laminated on a heated tin free steel and the resulting film-laminated metal plate is processed to give a metal can.
The use of the thermoplastic resin film resolves the above-mentioned problems of simplification of the process, prevention of pollution and the like.
Of the thermoplastic resin films, for example, when a polyolefin film, such as polyethylene and polypropylene, is used, heat history from a can forming process or heat history from a retorting treatment after can forming and the like may cause peeling off of the film from the film-laminated metal plate, because this film has poor heat resistance.
A method comprising the use of a polyester film as the thermoplastic resin film is most preferable, because the problems associated with the above-mentioned polyolefin film can be resolved.
A polyester film applied to the inside of a can shows superior heat resistance and allows only a small level of a low molecular weight substance to be generated. Consequently, it show less occurrence of spoiled taste and flavor of food due to the transferred low molecular weight substance, as compared to a polyolefin film. In other words, this film is superior in preventing occurrence of spoiled flavor.
However, the use of a polyester film containing polyethylene terephthalate as a main component for this end is not free from a problem. This is because a heat treatment during a can forming process after laminate processing, which aims at superior finish of the can, covering an uncovered joint part with a film band, and the like could cause dimensional changes only in the film part of the film-laminated metal plate due to the insufficient heat resistance of the polyester film, which in turn results in a loose film and a failure to completely cover the surface of a metal plate.